This invention relates generally to electromagnetic interference (xe2x80x9cEMIxe2x80x9d) shielding and, more specifically, to shielding cable connection ports from the transference of EMI therethrough.
As used herein, the term EMI should be considered to refer generally to both EMI and radio frequency interference (xe2x80x9cRFIxe2x80x9d) emissions, and the term electromagnetic should be considered to refer generally to electromagnetic and radio frequency.
During normal operation, electronic equipment generates undesirable electromagnetic energy that can interfere with the operation of proximately located electronic equipment due to EMI transmission by radiation and/or conduction. The electromagnetic energy can be of a wide range of wavelengths and frequencies. To minimize the problems associated with EMI, sources of undesirable electromagnetic energy may be shielded and/or electrically grounded. Shielding is designed to prevent both the ingress and egress of electromagnetic energy relative to a housing or other enclosure in which the electronic equipment is disposed. Since such enclosures often include vent panels and gaps or seams between adjacent access panels, around doors, and at cable connection ports, effective shielding is difficult to attain, because the gaps in the enclosure permit the transference of EMI therethrough. Further, in the case of electrically conductive metal enclosures, these gaps can inhibit the beneficial Faraday Cage Effect by forming discontinuities in the conductivity of the enclosure which compromise the efficiency of the ground conduction path through the enclosure. Moreover, by presenting an electrical conductivity level at the gaps that is significantly different from that of the enclosure generally, the gaps can act as slot antennae, resulting in the enclosure itself becoming a secondary source of EMI.
Specialized EMI gaskets have been developed for use in shielding small gaps in electronic enclosures. These include, but are not limited to, metal spring fingers, wire mesh, fabric-over-foam, and conductive elastomers. To shield EMI effectively, the gasket should be capable of absorbing or reflecting EMI as well as establishing a continuous electrically conductive path across the gap in which the gasket is disposed.
One particularly challenging shielding issue on electronic enclosures is cable connection ports. In most instances, an electronic circuit disposed within an EMI shield requires interconnections with one or more external sources and/or destinations. Consequently, the shield provides interface ports, such as cable connection ports, to allow communication therethrough. Exemplary interfaces include power leads and signal cables. To maintain the integrity of a shield, prior art solutions use shielded cable. A shielded cable generally includes one or more signal and/or power leads substantially surrounded by a conductive jacket. Ideally, the conductive jacket is in electrical communication with the shield, thereby becoming an extension of the shield to the remote source/destination. Depending on a desired level of shielding effectiveness, and the wavelengths of the EMI, the conductive jacket may be one or more electrically conductive braids, an electrically conductive foil, and even an electrically conductive conduit (i.e., a pipe).
Some applications, however, require that a shielded electronic circuit be electrically isolated from its interfacing source/destination. Conductive shields generally preclude such electrical isolation, as they are often used to extend one electrically conductive boundary to another. One solution allowing such electrical isolation is an optical interface, such as a fiber optic interface. It is common for networked computers and other electronic devices to have multiple optic-to-electric transceivers and other electronic devices attached to circuit boards. Typically, non-conductive, plastic bulkhead connectors attach fiber optic cables to a circuit module containing a circuit board having at least one optic-to-electric interface.
Referring to FIG. 1, shown is a representative portion of a circuit module 45 connected to a fiber optic cable 50 within a networked computer system. The portion of the circuit module shown in FIG. 1 includes a circuit board 55 attached to a fiber optic device, such as a high-speed fiber optic transceiver 60, and a fiber optic pigtail 65. A fiber optic cable 50 external to the circuit module 45 connects to the fiber optic pigtail 65 through an aperture 70 in a faceplate or bezel 75. Typically, a non-conductive, plastic bulkhead connector 80 extends through the faceplate 75 to connect one end of the fiber optic cable 50 to the fiber optic pigtail 65. One or more EMI gaskets 85 are provided between the faceplate and adjacent modules.
Generally, circuit modules have multiple bulkhead connectors to service multiple fiber-optic cables representing multiple channels. Each bulkhead connector requires a mounting aperture, or hole, typically on the order of 0.5 inches square in the faceplate 75 covering the circuit module 45. Unfortunately, these holes are large enough to pass considerable EMI through the shielding barrier formed by a row of faceplates.
One prior art solution to limit the amount of interference passed to the transceivers is to have the fiber optic cables pass through a set of compliant compression flanges that sandwich the cables. See, for example, shielding devices described in U.S. Pat. No. 6,162,989, entitled xe2x80x9cCable Entry Shield (EMI-RFI) for Electronic Unitsxe2x80x9d issued to Garner, the disclosure of which is herein incorporated by reference in its entirety. Another proposed solution is to have cables pass through slits in an electrically conductive cloth. See for example, a shielding device described in U.S. Pat. No. 6,101,711 xe2x80x9cMethod for Reducing Electromagnetic Waves Radiated from Electronic Devicexe2x80x9d issued to Kobayashi, the disclosure of which is herein incorporated by reference in its entirety.
One problem with the shielding devices described in these patents is that these devices do not sufficiently shield the openings around bulkhead connectors, especially bulkhead connectors with varying dimensions. Additionally, gaps around the cables may still transmit emissions, which are then passed to sensitive circuitry. These gaps that may have proved effective in the past are becoming unacceptable in view of the trends in electronic circuits to operate at higher speeds and greater sensitivities.
Another prior art solution to limiting the amount of interference passed to the transceivers is to externally cover the bulkhead connectors within an externally mounted shielding device (i.e., a xe2x80x9cbootxe2x80x9d). See, for example, externally mounted shielding devices described in U.S. Pat. No. 6,158,899, entitled xe2x80x9cMethod and Apparatus for Alleviating ESD Induced EMI Radiating from I/O Connector Aperturesxe2x80x9d issued to Arp et al., the disclosure of which is herein incorporated by reference in its entirety.
One of the problems with the externally mounted shielding devices described in Arp et al., is that these devices are too cumbersome and take up too much space to enclose and shield all of the cables in a networked computer system.
Accordingly, it is an object of the invention to provide a compact, EMI shielded interface, such as a cable connection port or access hole, for attenuating the transference of EMI therethrough over a wide range of frequencies (e.g., above 109 Hz).
In one aspect, the invention relates generally to a device for reducing transference of EMI across a conductive boundary defining an aperture in a structure, such as an equipment enclosure or faceplate. The device includes a conformable member having a conductive external surface extending along at least a portion thereof. The conductive external surface is in electrical communication with the conductive boundary. The conformable member also defines a first conductive channel of a predetermined minimum length extending therethrough. The first conductive channel is in electrical communication with the conductive external surface and is adapted for receiving at least a portion of a cable assembly. The first conductive channel has a proximal end having a first aperture and a distal end having a second aperture. The second aperture has a predetermined maximum cross-sectional dimension, such as a diameter, that is less than about one half of a predetermined cut-off wavelength. An attenuation relating to the transfer of EMI through the second aperture is determinable according to the maximum cross-sectional dimension and the minimum length.
In one embodiment the first conductive channel is a bore. In another embodiment the first conductive channel is a groove. In yet another embodiment, the first conductive channel includes a groove and a conductive member, the conductive member and the groove forming, when mated, a bore.
In one embodiment, the conformable member includes a first conformable element and a second conformable element. When mated, the first and second conformable elements form a conductive bore extending therethrough.
In some embodiments, the conformable member includes conductive flexible polymeric material or a conductively coated flexible polymeric material. In other embodiments, the conformable member further defines a second conductive channel, also adapted for receiving a portion of a cable assembly. The second conductive channel includes a third aperture at a proximal end and a fourth aperture at a distal end. As with the second aperture, the fourth aperture has a predetermined maximum cross-sectional dimension less than about one half of a predetermined cut-off wavelength. In one embodiment, the first aperture and the third aperture are the same, common aperture.
In some embodiments, the conductive member is disposed on a first side of the aperture defined by the conductive boundary. In other embodiments, at least a portion of the conductive member extends through the aperture. In still further embodiments, the proximal end of the conformable element is adapted to conform to an angled surface.
In some embodiments, the distal end portion of the channel has a length at least as great as the maximum cross-sectional dimension thereof for attenuating EMI signals. In one embodiment, the distal portion of the channel has a length at least two times greater than the maximum cross-sectional dimension of the aperture.
In another aspect, the invention relates to a process for reducing a transfer of EMI across a conductive boundary defining an aperture in a structure, such as an equipment enclosure. In one embodiment, the process includes providing a device as described above for reducing the transference of EMI across the conductive boundary defining the aperture. In another embodiment, the process includes adapting the conformable member to mate with a portion of a circuit board or other proximate structure.
In another embodiment, the process includes adapting the conductive member to function as a grommet, at least a portion of the conductive member extending through the aperture defined by the conductive boundary. In yet another embodiment, the process includes adapting the proximal end of the conformable element to conform to an angled surface.
In yet another aspect, the invention relates to a device for shielding EMI, whereby the device includes a conductive element forming an aperture. The aperture is adapted to receive a cable. In one embodiment, the conductive element includes a first end having a conductive gasket adapted to contact a support structure and a second end including a conductive gasket adapted to contact a circuit board. The conductive element also includes a conformable element positioned partially within the aperture and adapted to receive the cable. The conformable element forms a channel extending therethrough, forming a waveguide having an aperture with a maximum cross-sectional dimension less than about one half of a cut-off wavelength. The waveguide attenuates EMI signals having a wavelength greater than the cut-off wavelength.